This revised application proposes to compare the activities of the wild type and variant versions of c-Myb expressed by normal cells and leukemias, and to determine their relevance to human disease. The c-myb proto-oncogene encodes a DNA binding transcription factor with latent transforming activity. Relatively minor changes to the c-Myb protein unleash its ability to cause leukemias in birds and rodents and microarray assays have shown that even minor changes to its C-terminal domains result in dramatic changes in transcriptional activity. Alternative RNA splicing generates multiple c-myb mRNAs that encode proteins with altered C-terminal domains and the alternative splicing is more abundant and more varied in leukemias than in normal cells, suggesting that variants of c-Myb may participate in leukemogenesis. In Aim 1, the structures of the most abundant variant c-myb mRNAs expressed in a representative group of normal hematopoietic cells and leukemias will be determined. To compare their activities, the normal and variant forms of c-Myb will be expressed in human monocytes and microarray assays will be used to identify telltale changes in gene expression. Aim 2 will test the biological relevance of the variant c-Myb proteins. First, alternative splicing of c-myb gene products will be studied in differentiating hematopoietic cells, then wild type and variant c-Myb proteins will be expressed in normal hematopoietic progenitors, to see if changes in differentiation or proliferation are induced. In Aim 3, quantitative assays will be used to follow the expression of normal and variant c-myb RNAs in a defined cohort of adult AML samples, and then statistical analyses will determine whether specific variants correlate with clinical parameters, prognosis or gene expression patterns in AML patient samples. The proposed experiments build on our expertise and extensive preliminary and published data and have great relevance to the study of alternative RNA splicing, the activity of Myb proteins and human leukemia.